The present invention relates to the field of illumination, and in particular to light tunnels used in optical systems such as illuminators to achieve uniform illumination.
Achieving uniform illumination is necessary in numerous optical applications, and is particularly important in the fields of microscopy, and the relatively new field of photolithography. Many illumination uniformization techniques have evolved over the years to meet the increasing demands on illumination uniformity. With the advent of the laser in the 1960""s, new techniques have been developed to deal with illumination non-uniformities arising from interference effects due to the coherent nature of laser light.
In certain applications, such as photolithography, materials processing and the like, it is desirable to illuminate an object with light having an intensity distribution that is both macroscopically and microscopically uniform. Here, macroscopic means dimensions comparable to the size of the object being illuminated and microscopic means dimensions comparable to the size of the wavelength of the illumination. In many of these applications, it is further desirable to use a pulsed laser source and to have a spatially uniform intensity distribution. However, the light output of a pulsed laser source is spatially non-uniform. Macroscopically, the light beam often has a gaussian-like cross-section (xe2x80x9cprofilexe2x80x9d). A great deal of effort has gone into fabricating lasers that emit a beam having a more uniform profile, but even these are only uniform to +/xe2x88x9210% over limited areas. As a result, it is often necessary to use auxiliary optics with a pulsed laser light source to make the illumination more uniform.
The challenge in producing a spatially uniform intensity distribution from a laser source arises from its inherent temporal and spatial coherence. When two incoherent light beams overlap, the intensities of the two beams add. However, when two coherent beams overlap, the electric fields of the two beams add, which can produce an intensity having an interference pattern comprising fringes not present in an incoherent illumination system. As a result, the traditional methods of producing uniform illumination with incoherent sources are typically unsuitable for coherent sources like lasers.
With reference to FIGS. 1A and 1B, there are shown schematic cross-sectional diagrams of conventional illumination uniformizer apparatus 10 and 70 for achieving uniform macroscopic illumination. The conventional uniformizer apparatus works well for incoherent (i.e., xe2x80x9cnon-laserxe2x80x9d) sources, but is inadequate for coherent (i.e., xe2x80x9claserxe2x80x9d) sources. For many applications, apparatus 10 of FIG. 1A comprises, along an optical axis A, a laser light source 16 emitting short pulses of coherent light L (e.g., 10 ns/pulse) comprising light rays R1 and R2, a condenser optical system 24, and a hollow light tunnel 30 with an interior region 32, upper and lower walls 36 and 40, respectively, and corresponding highly reflective inner surfaces 36i and 40i and outer surfaces 36o and 40o respectively. Light tunnel 30 further includes an input end 50 adjacent optical system 24, and an output end 56 at the distal end of tunnel 30 from optical system 24. A material often used for walls 36 and 40 of hollow light tunnel 30 is quartz, which is often coated with a high-reflectivity material such as a metal or a dielectric.
With reference to FIG. 1B, apparatus 70 includes the same elements, except that instead of hollow light tunnel 30, apparatus 70 includes a solid light tunnel 80 having an index of refraction n1, upper and lower surfaces 86 and 90, an input end 94 and an output end 98. A material often used for solid light tunnel 70 is fused quartz, which has a refractive index of about 1.5 in the visible wavelengths. Apparatus 10 and 70 are commonly used with incoherent sources to achieve better than +/xe2x88x921% uniformity at their respective output ends 56 and 98.
Because of the coherent nature of light source 16, intersecting light rays R1 and R2 passing through the light tunnel produce a light intensity distribution in the form of a standing sinusoidal wave pattern Ps at the output ends 56 and 98 of light tunnels 30 and 80, respectively. Here, two rays R1 and R2 and a central ray RS are shown for the sake of illustration. The period of standing wave pattern Ps is determined by the wavelength of the laser light and the angle between intersecting light rays R1 and R2, between rays R1 and RS, and between rays R2 and RS. In practice, there are many pairs of intersecting light rays (depending on the number of reflections), with each pair producing a standing wave pattern. The length and width of light tunnels 30 and 80 define the angle between intersecting rays R1, R2, and RS and the path length difference (i.e., the phase) between the intersecting rays determines the relative position of the irradiance maxima in standing wave pattern Ps.
A prior art technique for eliminating interference effects (e.g., standing wave pattern Ps) to achieve uniform illumination using a light tunnel is the breaking of the coherent light into packets and adding the packets incoherently, or by rotating a random diffuser between the light source and the light pipe entrance.
There are several U.S. patents directed to such techniques for eliminating interference effects that are relevant to light tunnel illumination systems. For example, U.S. Pat. No. 4,744,615, entitled xe2x80x9cLASER RAY HOMOGENIZER,xe2x80x9d describes a coherent laser ray having a possibly non-uniform spatial intensity distribution that is transformed into an incoherent light ray having a substantially uniform spatial intensity distribution by homogenizing the laser ray with a light tunnel. When the cross-section of the light tunnel is a polygon (as preferred) and the sides of the tunnel are all parallel to the axis of the tunnel (as preferred), the laser light at the exit of the light tunnel (or alternatively at any image plane with respect thereto) has a substantially uniform intensity distribution and is incoherent only when the aspect ratio of the tunnel (length divided by width) equals or exceeds the co-tangent of the input ray divergence angle theta and when Wmin= greater than 2RLcoh, where Wmin is the minimum required width for the light tunnel, Lcoh is the effective coherence length of the laser light being homogenized and R is the chosen aspect ratio for the light tunnel. This approach restricts the ratio of the tunnel""s length to width and consequently, the number of bounces for the light rays. However, the number of bounces affects the xe2x80x9cmacro-uniformityxe2x80x9d of the output of the tunnel. As a result, this approach can impact the macro-uniformity at the output of the homogenizer tunnel.
U.S. Pat. No. 5,224,200, entitled xe2x80x9cCOHERENCE DELAY AUGMENTED LASER RAY HOMOGENIZER,xe2x80x9d describes a system in which the geometrical restrictions on a laser ray homogenizer are relaxed by using a coherence delay line to separate a coherent input ray into several components each having a path length difference equal to a multiple of the coherence length with respect to the other components. The components recombine incoherently at the output of the homogenizer, and the resultant ray has a more uniform spatial intensity suitable for microlithography and laser pantogography.
U.S. Pat. No. 4,511,220, entitled xe2x80x9cLASER TARGET SPECKLE ELIMINATOR,xe2x80x9d describes an apparatus for eliminating the phenomenon of speckle with regard to laser light reflected from a distant target whose roughness exceeds the wavelength of the laser light. The apparatus includes a half plate wave member, a first polarizing ray splitter member, a totally reflecting right angle prism, and a second polarizing ray splitter member, all of which are in serial optical alignment, that are used in combination to convert a linearly (i.e., vertically) polarized light ray, which is emitted by a laser having a known coherence length, into two coincident, orthogonally polarized, rays that are not coherent with each other, and that have an optical path difference which exceeds the known coherence length of the emitting laser, to eliminate the speckle.
U.S. Pat. No. 4,521,075, entitled xe2x80x9cCONTROLLABLE SPATIAL INCOHERENCE ECHELON FOR LASERxe2x80x9d, describes a system for achieving very uniform illumination of a target. A ray of broadband spatially-coherent light is converted to light with a controlled spatial incoherence and focused on the target. An echelon-like grating breaks the ray up into a large number of differently delayed raylets with delay increments larger than the coherence time of the ray, and a focusing lens overlaps the raylets to produce at the target a complicated interference pattern modulated by a smooth envelope that characterizes the diffraction of an individual raylet. On long time scales, compared to the coherence time, the interference pattern averages out, leaving only the smooth diffraction envelope. This approach only works for a sufficiently long time duration and therefore limits the laser pulse length. This may not be an acceptable solution for some applications.
In sum, the above described prior art techniques are either too complex to apply to light tunnel systems, or are unduly restrictive in their application.
The present invention relates to the field of illumination, and in particular to light tunnels used in optical systems such as illuminators used to achieve uniform illumination. The present invention solves the above-described uniformity problems by reducing or removing the effects of standing wave patterns by laterally shifting the standing wave pattern at the output end of the light tunnel at high speed by actively shifting the boundaries of the light tunnel using an acousto-optic (AO) modulator.
Accordingly, a first aspect of the present invention is a light tunnel apparatus having an output end for uniformizing light traveling through the light tunnel. The apparatus comprises a light tunnel having first and second sides, and one or more AO modulators respectively arranged on at least one of the first and second sides. The AO modulators are arranged such that their activation causes at least one of the first and second sides to be displaced. This displacement changes the path of light traveling through the light tunnel by an amount sufficient to reduce illumination non-uniformities at the output end. The light tunnel may be hollow with reflective inner surfaces, or a solid light tunnel made from transparent material with a refractive index greater than 1.
A second aspect of the invention is an illumination uniformizer apparatus comprising, in order along an optical axis, a light source (e.g., a laser), a condenser optical system, and the light tunnel apparatus of the present invention as described above.
A third aspect of the present invention is a method of uniformizing light traveling through a light tunnel having first and second sides and an output end. The method comprises the steps of first, injecting light into the light tunnel. The next step is then displacing at least one of the first and second sides by injecting acoustic energy into the light tunnel through at least one of the first and second sides. This second step may involve driving an AO modulator at a frequency of 100 MHz or greater. The light traveling through the tunnel comprises light rays having a path length which, depending on the exact nature of the path, can vary by a half wavelength or more due to the modulator. Preferably, the displacement of the one or more sides is such that interfering light rays are imparted with a path length difference greater than half the wavelength of the light rays.